In Unit 3 we examined how populations change
over time. Sometimes the presence of another species, or sometimes the abiotic
environment itself affects a species. But the focus of the unit was on the
individual, and on populations of a given species. In this unit we are going to
switch our level of investigation to the community level. Instead of examining
a single species, we are going to examine how species interact to create a complex
network of interdependent relationships.

A good example would be a comparison with
human communities. You live within a community where you do business: your
bank, the grocery store, your school, and the restaurant where you work. At
each place you have a specific type of interaction that allows you to live
within this community. A change in one relationship, such as an increase in
tuition fees, may affect the amount of time you now spend at the restaurant.

Organisms have different types of relationships
with other organisms that share their habitat. Some of these relationships are
beneficial, while others are not. Humans become keenly aware of these complex
interactions when we alter ecosystems. Adding or removing key species can set
off a chain of reactions that can permanently damage an ecosystem.

References to sources are indicated by
numbers in parentheses [example: (3)] in the text. The list of sources is at
the end of the unit before the study questions.

Read "What is a
community " on pp. 11.59

Unit 4 outline:

4.1 Introduction

l. definition of a community
ll. species richness
lll. species diversity

A community can be defined as a group of
organisms that interact. This is different from a population, which is a group
of individuals of the same species. Communities may exist on a small scale,
such as an suburban garden with, or a large scale such
as the Brazilian rain forest.

ll. species richness

There are different ways to examine variation within a community. One is to
examine the total number of species that a community contains. This measure is
known as species richness. Species richness has been investigated for many
years and a number of conclusions can be made. One is that tropical communities
are richer in species than temperate communities. For example, the number of ant species that exist in different regions of the
world appear to vary with latitude:

Location

Species Number

Degrees Latitude

Alaska

7

60 N

Iowa

73

45 N

Cuba

101

20 N

Trinidad

134

10 N

Brazil

222

15 S

Topographic features such as peninsulas,
however, can diminish the richness in a local area by reducing the chances of
migration.

There are many hypotheses that attempt to
explain this variation in species richness, but why should we care? Because if we can understand the factors that promote and protect
diversity, then we can use our conservation efforts more efficiently to
preserve global biodiversity. One million dollars spent to protect a
rainforest with 50,000 beetle species may be a more prudent investment than
protecting a taiga environment with only three types of beetles. In general,
explanations for species richness fall into one of two categories: ones that
are biotic (organismal) based and those which are abiotic (nonorganismal)
based.

Biotic hypotheses state that it is the
interaction among the species themselves and the complexity of the environment
that promotes diversity in the tropics. For example one hypothesis, called the
animal- pollinator model, argues that the winds are less frequent and intense
in the tropics. Dense vegetation helps diminish the winds as well. Thus most
plants in the tropics are animal pollinated (bats, birds and insects).
Coevolution of the plants and animals in this region has resulted over time in
an increase in pollination efficiency, thus leading to a high degree of
organismal specialization. As more and more specialists evolve, more habitats
exist, and thus more species can co-exist (Waser, 1996)(1).
While this may hold true for differences among terrestrial environments it does
not explain any differences that may exist among aquatic ecosystems. Thus the
animal-pollinator model can explain not all variation in species richness.

Alternative explanations are based on
abiotic factors. These explanations suggest that differences in environmental
characteristics (instead of organisms) drive the system and result in parallel
changes in species richness. For example the abiotic productivity model
suggests that the greater the productivity of a region, the greater the amount
of plant and animal material that can be produced and thus the greater the
number of niches and species which can be supported.

There are over 28 species richness models in
all. Each one has its supporters, its critics and has produced conflicting
evidence. It is still unclear whether one model- biotic or abiotic, can explain
all the patterns seen in the living world. Instead different combinations of
models may explain diversity in each biomes. Whatever
the ultimate factors are that promote species richness,
conservation which preserves areas with the greatest number of individual
species will also protect biodiversity.

lll. species diversity

A problem exists when only using the number
of species to describe an ecosystem- it does not take species abundance into
account. For example, it may be important to know not just that an ecosystem
contains tigers but also how many tigers exist. This may help us determine the
health of an ecosystem. Species diversity is a better measure than species
richness because it incorporates both the number, and abundance of a given
species. Abundance is the number of individuals per a given species.

A number of indices have been created to
measure species diversity. Recall that an index is a relative measure of
comparison. Say you wish to compare the sweetness of three oranges. You create
an index of sweetness that ranges between (bitter) 0 and 1 (sweet), and rank
each orange accordingly. A ranking of .9 means nothing unless you realize that
the index only goes to 1, and thus your orange is very sweet. Indices can also
be created to measure how many different animals exist within a community. Two
examples will suffice. One group of indices, called dominance indices, is
primarily influenced by the number and abundance of common species and less by
the number and abundance of rare species. This bias may have drastic
consequences when these indices are used by governmental agencies. For example,
park managers who use this method may conclude that an ecosystem is maintaining
its diversity when its rare species are actually dying out. Thus we need to ask
the question: should all species be counted equally, or should rare species
(such as Bengal tigers) be given more
"weight" than common species such as house sparrows? Ordinal indices
are ones that rank the importance of a species as well as its abundance in the
environment. This would make bumble bees, which are
widely distributed on a global basis, less important than mountain gorillas,
which have a very small geographic distribution.

There is no perfect index of species
diversity- each has its own built in assumptions. Choosing the most appropriate
measure may depend upon the types of questions being addressed. For example,
conservation biologists would be prudent to use ordinal indices, which favor
rare species, when trying to preserve endangered species habitat.

No matter what the actual number of species,
species will end up interacting with one another. Everything from two species
sharing a burrow to a lion eating a mouse would be considered an interaction.
We can divide the types of interactions that exist among species into those
that have a positive effect, those that have a negative effect, and those that
have a neutral effect upon the participants.

Type
of Interaction

Effect on Species 1

Effect on Species 2

Predation

+

-

Competition

-

-

Mutualism

+

+

Commensalism

+

0

In general, when the effects of an
interactions are particularly positive (leading to an increased reproductive
success among individuals) or negative, (leading to a decreased reproductive
success among individuals), this selective pressure favors traits that lead to
coevolution.

l. coevolution

Coevolution can be defined as a genetic
change in two species as a result of their interactions with one another. When
individuals benefit or suffer from interacting with another species, those
individuals who reap the most benefit (such as the harvesting of energy) or
suffer the least amount of harm (by escaping predation) are more likely to
survive and reproduce, adding their genes to the next generation of
individuals. Over time the frequency of a trait may increase or decrease in a
population as a direct result of the interspecific interaction.

Coevolution is well documented in the
literature. One of the best known systems is that of figs (genus Ficus) and fig
wasps. Over 900 species of Ficus exist, and each one is pollinated by its own
species of fig wasp. In return for pollination, the female wasps have a place
to lay their eggs. The interaction between the two species is so close that
male wasps never even leave the fig. Once they are hatched, they locate the
females within the plant and inseminate them. The males then die. When Smyrna figs were introduced from Turkey into California in the late 1800's the crops
repeatedly failed until the obligate pollinating wasp species was introduced as
well.

Occasionally coevolved species attempt to
"cheat" on one another. Orchids are often pollinated by specific
species of bees. Some orchids even mimic female bees: males pick up and
transfer pollen while attempting to copulate with the flowers. In one species
of orchid (Ophrys) males prefer the flowers to real female bees. While this is
highly advantageous to the orchids (since males spend a lot of time visiting
them and transferring pollen) it is highly disadvantageous to the bees that
fail to reproduce! In contrast, some species of male Bombus bees
"cheat" by biting through the flower petals to get the nectar and
avoid entering the flower and picking up the pollen.

ll. predator-prey

The first type of interaction we will
examine is that in which one species greatly benefits at the expense of a
second species. These are generally called predator-prey relationships and can
be broken down into four basic types:

1) Carnivory: Animals feeding on herbivores or other carnivores.
2) Cannibalism: Where predator and prey are of the same species.
3) Herbivory: Animals feeding on plants.
4) Parasitism: Animals or plants feeding on other organisms without (typically)
killing them.

There are a variety of strategies that organisms that have evolved in organisms that reduce
their risk of being eaten. This suggests that predation is a strong selective
force. Listed below are some of the most common traits found in prey species
that reduce the risk of predation:

1) Aposematic coloration:
coloration that advertises a distasteful or toxic prey. Ex. Monarch butterflies
accumulate poison from milkweed plants and are consequently distasteful to
birds and other predators.

2) Mimicry: animals that
look, sound, or behave like another animal. Mullarian mimicry occurs when one
distasteful species looks like another. This reinforces the basic noxious
"appearance" of both species. An example where some butterflies and
wasps have coevolved to look like one another. Batesian mimicry occurs when a
tasty or harmless species evolves to mimic a noxious species. An example would
be venomous coral snakes and the innocuous milk snakes, which mimic the coral
snake color patterns. Note that this type of mimicry only works when the
"tasty" species is less common than the noxious species. Under these
conditions, predators who select a prey are more likely to bite into a noxious
species and be deterred from future nibbling.

3) Crypsis: is the
development of a "frozen posture" so that the prey species is
camouflaged against its background. Often the appendages are retracted to avoid
detection. Ex. Chameleons whose skin tone can be adjusted to match the
background.

4) Intimidation displays:
behavior on part of the prey species to decrease the likelihood of a predator
attacking. Ex. Toads swallow air to appear larger.

5) Polymorphisms: when genes
arise in a population such that there is more that one distinct physical type
(morph) within a prey species. Ex. Green and red morphs of pea aphids can be
maintained by the action of predators. Predators who
selectively prey on green bugs are less likely to go after the red individuals.
Thus the frequency of green individuals will go down, while the frequency of
red individuals goes up. Note that at some point the predator (who is having a
hard time finding green individuals) will begin to prey on red individuals and
the green morphs will begin to increase in frequency, and so on….

6) Chemical defenses:
chemicals that can be excreted to ward off potential predators. Ex. Toads have
salivary glands called parotid glands located in their head region. These
glands secrete noxious substances onto their body surface when the toads are
disturbed by predators. Sometimes these chemicals can make the predator
physically sick or mentally impaired. Once the predator learns to associate the
prey the noxious taste, it is often a lifetime association. Just think back to
a food you really hate and think why! Often it is because you got sick on that
item and have developed a strong aversion for that food item.

7) Masting: the synchronized
production of all prey young within a short time period such that a predator is
satiated and cannot eat all of the young. The result is that some young always
manage to survive to reproduce. Ex. 13 - year and 17- year cicadas emerge all
at once. There are too many insects to eat at once. As a
result, some cicadas to escape predation and survive to reproduce. Many
species of predators have coevolved in response to this feeding opportunity and
may time the production of their own young to capitalize on this bonanza. In
birds of prey, such as great-horned owls, eggs hatch in January. The young
raptors are now ready to feed on the young of prey species that hatch in the
spring.

How common are these defense mechanisms?
Witz (1989)(2) surveyed 354 papers, focusing primarily
on insects. He found that the most common antipredatory defense mechanism used
by prey was chemical defense (46%). Studies of natural systems indicate that
the effects of predation are significant and have greatly influenced the
evolution of prey species. Thus coevolution is likely to be a strong force in
predator- prey relationships.

ll. herbivory

Herbivory is the act of an animal, whether
invertebrate or vertebrate, consuming a plant. The variety of plant defenses
which exist suggests herbivory, like predation, is a strong selective force
that shapes plant evolution over time. Plant defenses can be divided into two
basic categories: chemical and structural.

Chemical defenses include nicotine and
caffeine, tannins and resins, toxic compounds such as atropine, (found in
deadly nightshade plants), and chemicals that mimic insect hormones to disrupt
insect molts. This means that insects can not properly develop and reproduce.
Many modern insecticides such as Logic, which we use to kill imported fire
ants, are based upon the defense systems found in nature. Logic affects ants by
sterilizing the queen. Since she can no longer produce offspring, the colony
eventually dies out. This is why Logic does not kill off the colony
immediately, but takes several weeks to have an effect. Since chemicals are
expensive for the plant to produce they are often allocated to the most
valuable tissues (such as new and tender growth) or are produced only in
response to herbivory. On average, a plant loses 7-10% of its vegetation to
herbivores. This amount of loss may be manageable for older, larger, mature
plants, but not to younger, smaller plants. Plagues of locusts and other
swarming insects can overwhelm even the biggest plants however, and reduce them
to stalks in a matter of minutes.

lll. parasitism

Both parasitism and herbivory are often
considered special subdivisions of predation. This is because the
"prey" species of a parasite of herbivore does not have to die as a
consequence of the interaction. None the less, parasitism in particular has a
powerful selective pressure on the evolution of species, including our own.

Up to 50% of all organisms that inhabit the
earth are considered parasitic. This includes species that feed on plants as
well as tapeworms and leeches. Recall that parasites typically feed on their
hosts without killing them. If a mechanism has evolved in a parasite that
allows it to shed its eggs or young to another host (to continue its life
cycle), the need to keep the original host alive becomes irrelevant. Under such
conditions the parasite can become deadly. Parasites, which are sub-lethal to
healthy adult individuals, may also be lethal to the young, sick or very old
individuals.

lV. competition

So far we have concentrated on interactions
where one species benefits at the expense of another. Competition is a
different type of interaction altogether. Here both species are interacting to
obtain resources, and the interaction can be highly detrimental to both
species. Competition interactions can be categorized as follows. Resource competition, is said to occur when organisms interact in
order to gain access to resources or mates. This type of competition is most
often seen in invertebrates. Interference competition occurs when organisms
interact with each other physically. In vertebrates this interaction has often
been ritualized into a sequence of escalating threat behaviors between
organisms on adjacent territories. The winner of the competition typically
takes the best of both territories while the loser either dies, leaves, or
takes a suboptimal territory. Interference competition can also occur among
plants. For example allelopathy occurs in plants where one plant produce
chemical substances that keep other plants growing near them. These substances
may be acids, and bases, or organic compounds that limit light and nutrients. Bracken
ferns (Pteridium aquilinum) are a common vascular plant found around the world.
It produces toxins that accumulate in the topsoil. These toxins then kill the
germinating seedlings of other plants, especially conifers. Recall that
conifers are larger than the ferns and if they established themselves they
would block sunlight and diminish the success of the ferns.

The competitive ability of any given species may vary with environmental
changes. Factors such as temperature, humidity and oxygen availability may
affect how well one species can do against another. For example, Park and his
colleagues found that the competitive ability of two species of flour beetles
(Tribolium sp.) was significantly influenced by climate.(3)

Temperature (C)

Humidity

Climate

Percent (%) Wins by T. confusum

Percent (%) Wins by T. castaneum

34

70

Hot-moist

0

100

34

30

Hot-dry

90

10

29

70

Warm-moist

14

86

29

30

Warm-dry

87

13

24

70

Cold-moist

71

29

24

30

Cold-dry

100

0

Tribolium confusum, which did well in dry
conditions against Tribolium castaneum, did more poorly in wet conditions.
Moreover, these abilities seemed to be exacerbated by warmer temperatures.

Reviews on the frequency of competition in
nature find that 55-75% of all species investigated exhibit some type of
competition. Here are 6 types of competition and their definition:

Resource Competition

1) Consumptive or exploitative: where
individuals compete for resources such as food and water.

2) Preemptive: where individuals compete for
space.

3) Overgrowth: where one species is
overgrowing or blocking the light for another species

Interference Competition

4) Chemical: competition which uses the
production of toxins (allelopathy)

5) Territorial: behaviors such as fighting
used to defend space

6) Encounter: temporary, infrequent
interactions directly competing for a specific resource. An example may be a
permanent water source which individuals fight over when prolonged drought
dries up intermittent water sources.

It turns out that exploitative competition
is the most common, occurring in 71/188, or 37.8% of the species studied (4).
Preemptive and overgrowth competition is most often used by sessile (e.g.
non-moving) organisms, while territorial and encounter competition is more
likely to be used by active mobile organisms. Chemical competition is used by
terrestrial plants and not by aquatic plants since chemicals become diluted and
ineffective in water.

V. mutualism

Species interactions that result in a net benefit
to both species are considered mutualistic. Classic mutualistic associations
include plants and their pollinators, such as birds, bats and insects.
Typically the animals gets a tasty nectar meal out of
the interaction (while the plant gets a chance to pick up pollen from another
plant and transport some of its own pollen to another member of its species).
In one species of euglossine bees, males do not collect food from the flowers
but collect fragrances, which they then turn into a sexual attractant for
females of their species.

Several generalizations have been made about
mutualistic interactions:

1) The need for mutualism decreases as
resource availability increases. Thus mutualism may have first evolved, not a
result of gracious altruism, but as a reciprocal parasitism where two species
"get the best of each other".

2) Mutualism is more common in stressful
environments.

3) In large populations, the benefits of
mutualism per individual are reduced.

Obligate mutualism occurs when the
relationship between two species has become so tightly linked that one species
can not survive with out the other.

Vl. commensalism

Commensalism occurs when one species
benefits from an interaction with another species, but the second species is
neither harmed nor helped. The effect on the second species is neutral. The
most common type of commensalism is a condition called phoresey, which is the
passive transport of one organism by another. An example of phoresey would be
sea anemones growing on the back of hermit-crab shells. The hermit crab is
unaffected by the hitchhiker, while the sea anemones benefits by being exposed
to new food resources. Note the distinction between different types of
interactions can be blurred. Some hermit crabs use the sea anemone as
camouflage. In this case the interaction would no longer be considered
commensalism but have changes into a case of mutualism.

4.3 How interactions affect community
structure

l. consequences of community interactions

It is important to recognize that
mutualistic (and commensalistic) interactions can have consequences that extend
beyond the two species to the community level. In other words these species do
not live in a vacuum but operate with in a web of species interactions. Pull on
the web in one location, and the effects of that tug can be felt in many
places. An example of this can be seen in a salt marsh community. Marsh elders
(Iva frutescens) and black grass (Juncus gerardi) exist mutualistically in salt
marshes. The interaction between the elders and black grass leads to a decrease
in soil salinity, and an increase in oxygen levels surrounding the plants. When
one of the mutualistic species- black grass is removed
from the salt marsh, the action affects other members of the community. Aphids that lay their eggs on the elders (who benefited from the
black grass, are unable to find suitable egg habitat. Aphids now decline
in number. Populations of Ladybird beetles that fed off of the aphids now
decline as well. Thus a cascade of events are
stimulated by the loss of mutualism.

ll. keystone species

As we have just seen, communities tend to
have complex interactions and the addition or removal of a given species can
have far reaching effects. But do all species have such an impressive impact on
their neighbors? Studies suggest no. It turns out that the presence of some
species within an ecosystem is much more important than the presence of others.
A keystone species is one whose affect upon a given ecosystem far outweighs
either its biomass or its geographic distribution. It is important to realize
that a keystone species is not the same as a dominant species, one that is
important due to its relative abundance. A lion may have greater effect on the
savanna than gazelles, which are more common. Keystone species can be predators,
prey, or species that in some way modify their habitat.

Certain starfish and sea otters have been
described as keystone predators. Another example is the large mouth bass found
in Northern U.S. lakes. The removal of bass
from a lake in Michigan
led to a significant increase in the abundance of planktivorous fish (e.g.
those that eat tiny one celled organisms), the disappearance of all large
zooplankton and the appearance of small plankton species. (5) Thus the entire
distribution of organisms was disturbed by the removal of one species. When the
large mouth bass were reintroduced to the lake, the lake returned to its
original state. Another good example is found with the reintroduction of wolves
into YellowstonePark. The presence of the wolves caused
changes in the populations of mountain lions coyotes, elk and raptors.

What happens to communities over time? Do
they sit there, and remain at the status quo with the same species eating one
another year after year? Or do they follow patterns of growth and decline
similar to the rise and fall of human cultures and civilizations. Do changing
ecological conditions, such as changes in sunlight availability and nutrient
availability over time affect which species thrive and which do not? Moreover,
what happens when density independent events such as floods, drought, fire, and
storms interrupt an ecosystem? Do the communities pick up where they left off,
with all the same species in place? And if they change, do they follow
predictable patterns of change?

l. definition

There is a whole division of ecology devoted
to the investigation of these questions. An ecological disturbance can be
defined as any event, whether biotic or abiotic, which disrupts a community and
its current structure. As with other things, disturbance can be small scale,
such as the introduction of a exotic plant into a
pond, or large scale such as the massive 1998 flooding in Central
America following Hurricane Mitch. Ecological succession is then
defined as the sequence of chance that emerges as a result of the disturbance.

ll. ecological succession

If you look at a forest that has been
heavily burned, you will notice that a strange pattern of life begins to
emerge. Soon after the fire, small grasses and abundant wildflowers may bloom
and cover the ground. As the flowers fade away taller grasses and small shrubs
may make an appearance along with the seedlings of small trees. Continue to
watch this patch of land over the years and you may see a forest eventually
return. This pattern of change is called ecological succession. Succession may
be defined the pattern of recolonization by organisms. It may or may not occur
after a disturbance. Disturbances can include events such as fire, storms,
overgrazing, and erosion. Primary succession occurs when there is an invasion
of plants into an area where no plants have gone before … (sorry, too much star
trek). Thus there is no soil for the plants to grow on, and colonization is
slow. It may take hundreds or even thousands of years to build up sufficient
soil. An example of this would be the many lakes created in North
America when glaciers receded at the end of the Ice Age. Primary
succession accounts for only a small fraction of succession on the earth today-
such as that following volcano eruptions and the spreading of the sea floor.
Over geological time however, primary succession accounts for the progression
of organisms on continents, islands and the ocean floor. Secondary succession
occurs when a disturbance destroys the organisms within an area but leaves the
soil intact. In a way, secondary succession is really a temporal
"blip" in a longer-term primary succession pattern.

Early views of succession believed that the
organisms were replaced through a process known as facilitation. This view
argues that the presence of the first organisms somehow prepares the
environment and makes it easier for succeeding organisms to inhabit the land. An
example would be an invading plant that fixes nitrogen in a nitrogen poor soil.
More nutrients are now available so that a small bush may be able to thrive.
The bush now provides shade and protection from herbivory for young tree
saplings, and so on. The most extreme form of facilitation is called
enablement, when the survival of a particular species depends upon the
colonization of an earlier species. Under such conditions communities tend to
follow very specific patterns of succession since only certain plants can arise
at any given time.

Another type of succession has been found
called inhibition. Here the presence of a first species actually prevents the
development of certain subsequent organisms. Thus whoever colonizes an area
first determines how a community will develop, and what species will be
present. A good example of this is found in European sand dunes. In general,
sand dunes begin to form around clumps of grass and then spread as different
species of grass invade the area and hold the sands in place. In Europe, sand dunes begin to form around clumps of marram
grass, followed, in sequence, by fescue (another grass), sand sedge, and sea
couch. Van der Putten and co-workers transplanted plants of each species into
pots containing soil from either it predecessor or successor. They then
observed whether a grass would thrive or fail in the soil from another species.
They observed the following results.

in Marram grass soil

in Fescue soil

in Sand sedge soil

in Sea couch soil

Marren grass

Failed

Failed

Failed

Fescue

Thrived

Failed

Failed

Sand sedge

Thrived

Thrived

Failed

Sea Couch

Thrived

Thrived

Thrived

Van der Putten argued that plants were
harmed by the soil-borne diseases of their successors, but not by the diseases
of their predecessors. Thus once a plant established itself in a community, it
rapidly out-competed its predecessor and thrived until its successor comes
along. Marrem grass can only thrive in new habitats that have not been
compromised by the pathogens of the other grasses.

Another example of inhibition can be found
with sunflowers- a mid-successional plant. Decaying sunflower leaves produce
compounds that inhibited the growth of early successional plants seedlings such
as Amaranthus. In contrast, sunflower leaves did not inhibit the growth of
three-awn grass, a species that typically replaces sunflowers.

A third type of succession has been
discovered. It is called tolerance succession and appears to be intermediate
between facilitation and inhibition succession. Here any species can start the
colonization process. Thus a grass seed and a beauty-berry bush seed could both
invade a new area and establish them selves. But once the new plants have
colonized the area, the process of succession proceeds in a somewhat reliable
fashion until a climax community is reached. A climax community is comprised of
species who are not easily replaced tend to dominate an older mature ecosystem.
For example, Gray Birch trees tend to dominate younger forests while Oak and
Beech trees tend to dominate older forests in the Eastern
United States. An example of this is presented in the following
table which describes the predicted percentage of tree species in a forest over
time, and the actual data from a 200-year-old forest in New Jersey.

Predicted Percentage of Different
Species of Trees after 200 years

Tree Species

0 years

50 years

100 years

150 years

200 years

Actual Data

Grey Birch

100

5

1

0

0

0

Black-gum

0

36

29

23

18

3

Red Maple

0

50

39

39

24

4

Beech

0

9

31

47

58

93

Note that ecosystems can show a mix of
succession processes, and herbivory, disease, and human interference can alter
patterns of succession. Thus stochasticity can have significant effects of the
species composition of an ecosystem.

llI. ecological stability

For a long time, researchers believed that
communities as a whole were stable and that disturbance only came from outside
forces such as hurricanes. That is unless there was a disturbance,
the community would remain the same indefinitely. Long term data records, such
as the historic annual bird counts, seemed to support this idea. But how can we
test the idea of inherent stability? Two different methods have been suggested,
depending upon what it is we wish to measure.

To measure how well a community resists
change we could

1) apply a force or
pressure (such as over watering a field, or introducing rabbits)

2) see if the
community changes

3) repeat the experiment in different communities
(or in our case different fields)

To measure how well a community bounces back
after a disturbance (what could be called community resilience) we could

1) determine a
stable point where the population levels of different species appear unchanging

2) apply a force or
pressure

3) measure the time
it takes for the community to return to its original stable point

4) repeat the
experiment in different communities

Unfortunately there is no easy answer, or
set of conclusions that arise from these experiments. There are however, some
patterns that emerge based upon the type of biome involved in the disturbance.
For example, lakes tend to be weakly resistant and weekly resilient since there
is no easy way to wash pollutants away. Rivers may not be resistant (since they
receive so much run off from the land), but are more resilient than lakes since
the moving waters can carry pollutants away faster.

IV. diversity and
stability

The traditional view, called the equilibrium
hypothesis, argues that most communities are stable, and the interspecific
forces such as parasitism, predation and competition help maintain predictable
number of species and individuals. One factor that would help maintain this
equilibrium would be the diversity of the community. For a long time people
believed that communities with more diversity should be more stable. This was
based upon the notion that with more diversity there would be more resources at
hand. Thus the community could remain stable because of all the possible
interspecific interactions and niches available. It is similar to the idea that
the more self sufficient you are as an individual - for example the greater
number of tools and resources you have, the more
resilient and thus stable you are.

Unfortunately the inherent belief that
diversity corresponds to stability is not supported by experimental evidence.
There is anecdotal evidence both for and against links of stability and
diversity but experimental manipulations do not show a strong correlation.

V. non-equilibrium model

What is the evidence for equilibrium with or
without diversity? A British woodland bird community was studied for 22 years
from 1971 until 1992. They found that the amount of variability in the
community increased the longer the community was observed. This may be because
over the span of 22 years, overall environmental variability happened to
increase. A more modern view, called the non-equilibrium hypothesis, argues
that disturbance is a frequent and naturally occurring phenomenon. As a result,
species composition is ever changing. There is no one stable point that a
community reaches and maintains since a constant state of change is the normal
pattern.

VI. intermediate
disturbance hypothesis

A modern, alternative approach to both of
these ideas is offered by the intermediate disturbance hypothesis. This
suggests that the most diverse communities, such as coral reefs and tropical
rain forests, are kept diverse because they have multiple disturbances. Instead
of destroying a community, frequent modest disturbances actually diversify a
habitat and allow a maximum number of species to thrive and reproduce. To study
this idea, Sousa carried out an elegant experiment in a marine intertidal zone.
He found that small rocks easily moved by waves had a mean density of 1.7
sessile plants and animals. Intermediate-sized boulders, which were
occasionally moved, have a mean species number of 3.7 species and large,
immovable boulders had a mean of 2.5 species. Were there fewer species on the
smaller rocks because they were disturbed more often or simply because they
were small and thus had less surface area? To test this idea, Sousa cemented
the small rocks to the ocean floor. Species density increased on the small
rocks, indicating that it was the level of disturbance, and not the rock size
that kept species numbers down (6).

Other studies like the one described above
have supported the intermediate disturbance hypothesis. Their only drawback is
one of scale: most intermediate disturbance experiments are carried out in small
patches of forests or intertidal zones. It is unclear whether these patterns
will exist in large ecosystems.

Biogeography is the study of the distribution
of species and entire communities over time. For example, some of the most
complete studies of ecological succession have occurred when volcanoes
completely denude offshore islands. One of the most famous examples was the island of Krakatau in 1883. All life on the island
was destroyed by the eruption. Scientist were then
able to observe each species that recolonized the island, and the effect the
new species had on the island's community. In less dramatic fashion, scientists
can observe the present distribution of species and show how their present
distribution reflects distant evolutionary history. The fact that Australia broke
off from Pangaea before the evolution of mammals explains why there are no true
mammals on that continent.

Because islands are small and often
isolated, they serve as good models for the study of biogeography. The term
"island" not only includes places like Krakatau,
but any habitat where the species of the "island" are surrounded by
unsuitable habitat. Mountains, rivers, deserts and canyons can all create
islands which, for all intense purposes, species cannot cross. Thus these mini
ecosystems allow us to observe community interactions in a simpler environment.
The hope is that the generalizations that arise from these "simpler"
systems can then tell us something about more complex environments.

ll. equilibrium and island biogeography

In the 1960's MacArthur and Wilson tried to
develop some generalizations about the factors that determine species diversity
on islands, Their hypotheses have undergone several modifications in the last
35 years as researchers have expanded on the original model. Here are some of
the basic generalizations:

1) The number of species on an island will
tend towards equilibrium. This is a result of a balance between the rate of
immigration, and the rate of extinction on the island (see figure 53.21 on page
1127 in Campbell).

2) The equilibrium number of species on any
given island will be determined by the island's size, and distance from a
potential pool of colonists. These factors determine the rates of migration to
and from the island.

3) While a particular species may come and
go from an island due to emigration or extinction, the total number of species
on the island should remain constant (hence the idea of an equilibrium).

4) Distance from the mainland to an island
affects not only the rates of colonization, but the rates of extinction. This
is because immigration of new individuals can slow down the rate of extinction
by replacing the individuals who die.

5) Rates of immigration and extinction will
be affected by the number of species already present on the island. The greater the number of species on the island, the less likely
that a new immigrant will represent a new species. Extinction rates will
increase as species number increases since of the competitive exclusion
principle indicates that no two species can occupy the same niche.

There are strong data to support the idea
that species richness increases with island size. For example, there are more
land plants on the larger Galapagos Islands and more amphibian, reptiles and
beetle on the larger islands in the West Indies.
There is little consistent data, however, to support the other predictions of
island biogeography, and there ideas remain controversial.

Read Concept 55.1"The
Biodiversity Crisis" on pp. 1209-1212

4.6 Conservation and Biodiversity

We just learned that biogeography is the
study of the distribution of species and communities over time. We approached
this topic from a theoretical point of view with the island hypotheses of
MacArthur and Wilson. Biogeography also gives us vital information about the
overall diversity of life on the planet. It is in the field of conservation
biology that biogeography can make its most important contributions. Recall
from unit 1 that conservation biology is the area of science that focuses the
management of biodiversity. And as biogeographers have discovered, we are in a
global crisis.

Crisis is an overused term these days. It
seems like every time you turn on the news you hear about a crisis in Russia, or Serbia, or someplace new. We have
become habituated to these extreme words. So what do we mean by saying there is
a biodiversity crisis and what does it matter?

Unit 1 states that there are over 1.5
million described species and up to 80 million species in total. The greatest
numbers of species have been located in the tropics, and in coral reefs. It is
now estimated that humans have artificially altered over 50% of the land
surface on the planet and we use over 50% of the accessible fresh water. That
is a lot of activity for one species. Not only do we take over land to build
houses, malls, and everything in between, but we alter geological and chemical
cycles as well (note these topics will be discussed at length in Unit 5).

So what? The reason that human activity is
important is that we are accelerating the loss of biodiversity through the
accelerated extinction rate of organisms. In other words our activities are
resulting in the extinction of entire species. Extinction is a natural process
and all species eventually go extinct or evolve into something new. Though out
evolutionary history species have come and gone. But up to now the overall
rates of extinction have appeared fairly constant. Now it is estimated that the
rate of species extinction may be 50 times higher than in any time in the last
100,000 years. In the tropics it is estimated that the rate of extinction is
1000 to 10,000 higher that the normal "background" rate. This change
appears incredible at first, and one may be suspicious of such high estimates.
But several independent sources are supporting these estimates:

1) 11% of the 9040 known
bird species are endangered

2) 680 out of 20,000 plant
species may be extinct by 2000

3) 20% of known freshwater
fish are endangered or already extinct

What are the causes of current species
extinction and how much is linked to human activity? There are three main
threats to species:

1) competition
by introduced species

2) habitat
destruction

3) overexploitation

l. introduced species

The following data have been collected on
484 extinct species and the causes for their extinction:

Causes of 484 Cases of Animal Extinction by Activity

Cause of Extinction

Percentage

Cause unclear

56%

Introduced animals

17%

Habitat destruction

16%

Hunting

10%

Other causes

1%

It turns out that the introduction of other
animals, and direct habitat destruction, such as deforestation are the primary
known factors. The fact that 56% of the cases were unclear does not mean that
human activity was unimportant in these cases. Indirect human activities such
as pollution can be important, but they are difficult to measure directly.

Introduced or "exotic" species can
mean many things. They may be the bushes growing in front of your apartment
building or the majority of flowers growing in your neighbor's yard. Indeed
most of plants and even grasses we have growing in our yards and cities are
completely exotic. The fire ants, which plague us each year, are an introduced
species. The effects from introduced species can take many forms:

1) Predation: Think about your cat that
gobbles up all the birds in the yard.

2) Disease and parasitism: American chestnut
trees, and European and American elm trees have all
suffered tremendous losses through disease. The State of California bans the import of many fruits
because they are worried they will carry the "medfly" a small insect
that can wipe out the citrus industry.

3) Competition: Introduced cats, rats, and
mongooses have accounted for 43.4% of bird extinctions on islands.

ll. fragmentation and metapopulations

Habitat destruction can occur in many ways:
agriculture, mining, forestry, urban development and environmental pollution.
Even if the entire forest or coral reef has not been destroyed, the habitat may
be so fragmented by human activity that the remaining pieces of habitat are
unsuitable to sustain life (see figure 55.7 on page 1161 in Campbell). Since organisms
may be unable to move between these fragments, and unable to survive on the
edges of these fragments, the chunks of environment resemble islands.
The rules of island biogeography that were discussed above now apply to these
fragments or islands of suitable habitat.

We can call these
populations of a single species which are separated metapopulations. The
quality of the habitat "island" that each metapopulation lives on may
vary significantly. A patch with high quality resources may be able to sustain
a metapopulation and produce more offspring that a poor quality
patch. A poor quality patch is more likely to go extinct, and will only
be repopulated if immigrants from a high quality patch find the island.
Furthermore, the more isolated a patch is, the more likely that the individuals
in the patch will be cut off from the larger genetic pool and become
genetically extinct. Cricket frog (Acriscrepitans) vocalizations, and female mating preferences were studied in
ponds in the Central Texas area. It was found
that males in different ponds (in this case a pond is equivalent to an
"island) evolved different frequency mating calls. Thus
2.2 cm long male from a pond in Bastrop.
TX had lower frequency mating call than a 2.2cm male from a pond in Wimberley.
Furthermore, females from each pond had different abilities to hear the male's
call. Females from the Bastrop
pond were more attuned to hear low frequency mating calls and females from the
Wimberley pond were more attuned to high frequency mating calls. This suggests
that over time, isolated metapopulations can evolve away from each other until
they reach the point where they no longer recognize each other as potential
mates.

As mentioned, habitat quality differences
can lead to reproduction rate differences between different patches. A source
habitat is one in which reproduction rates exceed mortality rates. This means
that this metapopulation can be used a source of migrants to new habitats. A
sink habitat is the opposite. Here the mortality rate exceeds the reproduction
rate and the population is in decline. It is vital to recognize which habitats
are source habitat and which are sink habitats if conservation efforts are
going to succeed. There is little use in reintroducing individuals into sink
habitats since the animals will die before the population can grow. It is estimated
that as little as 10% of metapopulations may be source habitats. Life tables
(see Unit 3) are necessary to distinguish the growing from declining
populations. Work on the Peregrine Falcon and the northern spotted owl are good examples of the interactions between source and
sink habitats.

4.7 - Human Impact on Ecosystems and
Communities

Every time a new shopping mall is built, a
new road is cut, or a park is created, we drastically alter the natural
environment. While building a mall certainly destroys almost all life, roads
and parks are also destructive. Even lands that were once used for agriculture
and then abandoned are no longer natural ecosystems. Restoration ecology is a
branch of ecology that attempts to return disturbed ecosystems to their former
natural state. Two examples of restoration ecology will demonstrate how
difficult yet important a process it is.

First, let's turn to the problem of humans
as competitors. We can out-compete almost every other organism for food, water,
and other necessary resources. We do this every time we take ranch land and
develop it into a subdivision. Animals who live on
this land may migrate to adjoining lands, or die if there is no place to go.
This is the case with the top trophic-level competitors here in the United States.
We have driven animals such as bears, wolves, bald eagles, condors, and
mountain lions to the brink of extinction. Not only do we take away their
lands, but we also kill them since we believe they compete for our cattle and
sheep. In the process of removing these top predators, however, we drastically
alter the ecosystem. An example of this was seen in the case of removing the
large mouth bass from Northern U.S. lakes. By
removing top carnivores, such as wolves and bears we cause the number of small
predators to increase dramatically. For example the numbers of opossums,
coyotes, and raccoons in Texas
have risen sharply. These mid-sized predators no longer compete with top
predators for food resources such as snakes, mice and squirrels. In addition
the number of mid-sized predators has increased because no one, in turn, is
eating them. The numbers of herbivores, such as deer, also dramatically
increases because their population size is no longer controlled by predation
(while coyotes may be able to prey on small deer, raccoons and opossums can
not). Instead deer under go huge population blooms and crashes as their food
resources fluctuates. This winter in Central Texas
is a case in point. Many white tail deer are currently dying in the Hill Country
due to the extended drought and human activity. A relatively wet winter last
year coupled with low predation caused an increase in deer numbers. The
drought, and the recent building explosion in Hays county,
leads to minimal grass in the Hill Country. As a result deer are either
starving, or coming up onto the road shoulders at night to eat grass that grows
in ditches and roadside culverts. An average of two or three newly killed deer
have been found each week on a ten-mile stretch of highway between Kyle and
Driftwood Texas this fall.

Restoration ecologists try to reintroduce
top predators to rebalance ecosystems. Wolves have been reintroduced to places
like YellowstoneNational
Park, and Arizona, while
California Condors have been reintroduced to Southern
California. The results have been mixed at best. Many ranchers do
not want the wolves in the area where they may threaten their livestock. They
not only shoot wolves that have wondered off the protected lands but illegally
shoot wolves within the parks and protected areas as well. Condors that have
been raised in captivity and then reintroduced into the wild are imprinted on
humans. This means that their natural fear of humans has been decreased and
instead they are conditioned to expect food from humans. Recently several
condors have broken into houses in the Southern California
in search of food. This may seem innocent until you realize that a full-grown
condor stands over 4 feet tall and they use their feet and beaks to rip open
carrion and screen doors alike. Homeowners have come in to find condors tearing
through the house.

One promising solution is to condition
predators (like wolves) before their release to avoid livestock. John Garcia
found that rats who have ingested food, and then have gotten physically sick
after ingesting the food, will avoid eating the same food again in the future.
Indeed rats will forgo eating then eat that same type of food again. This
behavior, called taste aversion conditioning, may be familiar to you. If you
have ever gotten sick on a food (for me it is hot oatmeal) you know that you
would rather starve than to eat the food. Even the thought of the food is
distressing! Researchers have been able to take sheep and cattle carcasses,
lace them with a poison and let wolves feed on the remains. Not only will these
individual wolves avoid sheep and cattle, but pups that have been feed by their
mothers (through regurgitation) develop the aversion as well. If predators such
as wolves can be trained to avoid livestock, perhaps ranchers can be persuaded
to tolerate these important creatures.

Restoration ecologists also attempt to restore complete habitats. For example
after strip mining a track of land is ruined, since most of its animals and its
topsoil have been removed. By reintroducing topsoil and planting native seeds,
the land can potentially be coached back into its natural state. A working
knowledge of succession is vital in order to bring back a portion of the
original grasses, shrubs, trees and animals. Some ecologists are worried that
this ability to restore land will be abused. Fragile ecosystems such as taiga
and tundra in Alaska
could be destroyed as lands are opened for pipelines and oil exploration.
Habitat such as the tundra can not be restored in a matter of years. Instead it
may take hundreds or even thousands of years. Large-scale restoration of an
ecosystem is immensely expensive, and it is unclear who would pay for the
rehabilitation. Even if the monies are available, restoration ecology is still
in infancy as a science and the techniques are modest at best. After the wreck
of the oil tanker the TorreyCanyon in 1967, some of
the clean up methods such as suction devises and scrapers did more damage to
the habitat then the oil itself. The Exxon Valdez disaster ten years ago is
another case in point.

1. Draw a rank abundance diagram for a
community in which one species comprises 80% of the individuals in a community.
One species comprises 10% of the individuals. One species comprises 5% of the
individuals. One species comprises 3% of the individuals, and the remaining
species comprises 2% of the individuals. Label each axis.

2. List and define all of the types of
interspecific interactions.

3. Name and describe the various types of
predation.

4. Describe at least two examples of
interspecific interactions that have lead to the evolution of coevolved
adaptations.

5. Name and describe the various types of
anti-predator defenses.

6. Describe the parasite-host relationship.
What is the typical function of a primary or ultimate host? What is the typical
function of the secondary or intermediate host or hosts?

7. Name and describe the two basic kinds of
interference competition. What is the competitive exclusion principle?

8. What is the ecological niche? How is the
fundamental niche different from the realized niche? Why does competition
sometimes favor species partitioning resources?

9. Name and describe the different types of
symbiotic relationships. Describe in detail one type of coevolved relationship
that benefits both participants.

10. Explain how predation can influence
community structure. Describe one example from your textbook of how predation
can influence community structure.

11. Explain how disturbance can influence
community structure. Explain why succession does not always lead to only a
single climax community.

12. Describe how fire influence community
structure in central Texas
woodland-grassland communities. How does the frequency of fire in this context,
influence the type of plant community that should dominate in one area?

13. Describe the various ways in which
grazing by domestic livestock influence natural ecosystems.

14. Explain the Non-equilibrium model of
community composition. How does it compare with the equilibrium model of
community composition?

15. Compare the dynamic equilibrium model of
community composition with the intermediate disturbance hypothesis in terms of
the intensity and frequency of disturbance. What role do keystone species have
in communities?

Perhaps the most fundamental
"need" of any organism is the need to procure food. Not just
calories, but a balanced diet sufficient in energy, vitamins, nutrients, and
trace minerals. Without this balance animals can not successfully grow and
reproduce. As you have seen in the last unit, obtaining these nutrients means
very different things for an oak tree and a bird of prey. Organisms are linked
by more than the fact that they eat each other. The very calories and nutrients
an organism requires are locked up inside the leaves and roots, or muscle and
fat of the prey species. The laws of thermodynamics demonstrate that the
transfer of energy is never perfect, and a large amount of energy is lost each
time one organism consumes another. Thus a smaller and smaller amount of energy
is available for the animals at the top of the food pyramid.

There is a finite amount of energy and
nutrients in the world. Thus it is imperative to cycle these nutrients through
the bodies of organisms and back into the earth and water if we are to keep
ecosystems in equilibrium. Not only is it important to keep nutrients moving,
but to keep the relative amounts of nutrients in balance. Too much nitrogen or
phosphorous in a habitat can have as damaging effects as too little.

Ecosystem was a term first created by a
British plant ecologist named Tansley in 1935. The term was created to define
an environment, which included both the organisms and complex physical forces.
Thus it included not only organisms like bacteria, plants, fungi and animals
but their abiotic world, and the energy and minerals that move through that
world. An ecosystem can be small- such as a rock crevice filled with water
after a rain, or as large as the Amazon forest. There are no definitive
boundaries for most ecosystems.

ll. trophic structure

The trophic structure is the set of dietary
relationships between organisms in an ecosystem. In other words- who eats whom. Typically these relationships are not a simple linear
chain. Instead a better analogy would be a food web or net where simultaneous
interactions occur between a number of species.
Organisms are classified by function levels rather than by species. Thus
bullfrog tadpoles are considered herbivorous since they eat algae while the
adults are ravenous carnivores.

lll. connectance

Connectance is a measure by which we can
determine the relative complexity of a food web. In other words, how many
species are interacting with one another simultaneously.
It is defined as

such that for any given number of (n), the total number
of potential interactions is

n(n-1)
2

The number of links one species has with
other species is called the linkage density (d).Linkage density is calculated
as follows:

Linkage
density (d) = the number of actual interactions
the total number of species

We can use a case history of insects on
pitcher plants to illustrate our point. A study of the food web of insects on a single pitcher plants 3 found a total of 19 species
visited the pitcher plant and 33 different interspecific interactions occurred.
Thus in the case of the pitcher plant food web:

and connectance is equal to 0.19. The linkage density is
therefore 33/19 = 1.74.

Linkage density describes how tightly
interconnected species are within a given community. The higher the linkage,
the more dependent species are upon one another. Thus the extinction of a
species in a tightly liked ecosystem may have profound repercussions on many
organisms. Moreover, if a pollutant enters the ecosystem, the higher the
linkage, the more destruction will occur. The lessons learned from the
pesticide DDT are an sobering example of what happens
when toxins enter a tightly linked ecosystem These will be discussed below when
we examine human impacts on the environment.

It is difficult to know which links in a
food web are most important. One way to investigate this question is to
determine how much energy flows between two links. An experimental approach is
to manipulate a link (ex. remove a predator) and watch the results.

An example of this can be found in Texas by studying the
Attwater's prairie chicken. Attwater's prairie chicken, a brownish
chicken-sized bird was once quite common in the Southern portions of Texas, and the species
numbered in the millions at the end of the 19th century. Now there are fewer
than a dozen birds alive and they are all in captivity. A reintroduction
program was started at the Attwater's prairie chicken refuge near Eagle Lake, Texas.
One important question refuge managers have been asking is to what extent do
great horned owls feed on the prairie chickens? Moreover, what would happen to
the populations of reintroduced birds if you first remove the great horned
owls? Great horned owls probably feed on the prairie chickens if the
opportunity arises. But in all likelihood, great horned owls also feed on
opossums and raccoons who prey on the chicken. Thus a
"simple" solution of removing the owls may have unseen and
unfortunate consequences.

lV. food chain and food webs

There are a number of generalizations that have been made about food chains and
food webs. Some of the most important are as follows:

1) Cycles where species A eats B who eats C
who in turn eats A are very rare.

2) The average proportion of top predators,
intermediate and basal species remains roughly constant in webs independent of
the number of species. This is due to the percentage of energy available at
each level (see Energy flow)

3) Linkage density is often constant except
for webs with very large numbers of species.

4) Omnivory (where an organisms feeds on
both plant and animals) is rare.

5) Food webs are more complex in complex
environments.

6) Top predators tend to be large and
sparsely distributed, whereas herbivores tend to be small and more abundant.

5.2 Trophic Levels

l. primary producers

We can now organize organisms into who eats
whom. Primary producers are defined as organisms that produce their own energy.
Autotrophs, which use the sun to photosynthesize organic molecules such as
glucose, make up the largest percentage of primary producers. Phytoplankton,
tiny one-celled plants that live in ponds and oceans, are a good example.
Chemoautotrophs, which turn chemicals into food, are another, rarer type of
primary producer. These organisms tend to live in deep-sea vents. Instead of
using sunlight, chemoautotrophs use sulfur and oil that bubbles up from the
earth's mantle. In general, the ability of any primary producer to produce food
is limited by sunlight, temperature, water and nutrient availability.

ll. primary consumers

Primary consumers, as the name implies, are
the organisms that eat the primary producers. This would include any
invertebrate or vertebrate that feeds off of plants. Snails, insects, zebras,
frugivorous bats and seed eating birds would all be examples of primary
consumers. You find primary consumers in every habitat- in other words every
place food is produced. Primary consumers are often generalists, and opportunistic.
They forage on different plant species as food availability waxes and wanes. A
deer that only eats tender young grass shoots in the springtime may forage on
twigs in the winter when grass is unavailable. Primary consumers may also feed
up and down the food chain: squirrels for instance may eat both acorns and bird
eggs.

lll. secondary and tertiary consumers

Secondary consumers are those that eat the
primary consumers. A spider who eats the aphid feeding
off of a pine tree would be considered a secondary consumer. The chickadee that
eats the spider is now the tertiary consumer. Often times the higher level
consumers are larger animals that have extensive territories or home ranges.
The large predatory cats, raptors (birds of prey), bears and wolves and even
humans can fall into this category.

lV. detritivores

This trophic level category is often over
looked. Detritivores are the organisms that feed off dead and rotting tissue-
whether it is a dead deer or a dead plant. For example, it is important to realize
how much primary producer vegetation lies uneaten on the ground in the form of
leaves, stems and roots. Any one who gardens knows that old crops that are
plowed back into the ground, or compost which is added to the soil, produces
far more fruits and vegetables than ground which has been stripped clean of
plant material. It is the detritivores, such as bacteria and fungi, which break
up dead plants and animals and return the raw nutrients back to the soil. The
rate of mineral availability is often determined by the decomposition rate of
detritivores. Warm moist conditions favor this rate. Thus the decomposition and
turn over rate of nutrients is much faster in the tropics.

5.3 Energy Flow

When an armadillo eats a grub, some of the
energy that was trapped in the grub's body is now available to the armadillo.
But most of the energy from the grub (90%) will be lost as heat, crumbs spilled, or incomplete digestion. Thus 90% of the calories
in the grub will be "lost " and only 10%
will be turned into muscle and tissue in the armadillo's body. Now only a small
fraction of the energy from insect can be carried up to the next level- the
red-tail hawk that eats the armadillo. Most energy is lost as energy moves
through trophic levels, and this tends to keep food chains (i.e. who eats who)
very short.

l. primary productivity

The bulk of life on Earth consists of
plants- only a small fraction (less than 1 % by weight) comes from animals.
Gross primary productivity is defined as the amount of energy fixed by photosynthesis.
This number is huge, but does not take into account the amount of energy the
plants need to use to respire, grow, and reproduce. Thus net primary
productivity is a more useful measure of productivity. Net primary productivity
is equal to the gross productivity minus the amount of energy used by the
plant. Measurements from different habitats suggest that the primary
productivity is about 50% to 90% of the gross primary productivity. Large
plants, such as trees that have more structure and surface area to support,
tend to have reduced net productivity. Harvesting raw plant material and
weighing the biomass is the simplest way to measure productivity. Two types of
loss must be incorporated into this measure of overall biomass: biomass lost to
primary consumers (in other words herbivores eating the plants) and biomass
lost due to death of the plant.

People have used various methods to measure
productivity in different ecosystems around the world. If you look in your book
at Figure 54.4 you can see there is a tremendous amount of variation in biomass
production. For example, algal beds and reefs are the most productive while
extreme deserts are the least. Why is this important? As human beings gobble up
more and more lands to live and play on, we are running out of fertile land to
grow crops. It is essential to protect productive habitats such as temperate
grasslands -not only for their diversity but also for the number of food
resources that we depend upon.

What are the limits to primary production?
Water is the limiting resource in most terrestrial environments. There is a
direct correlation between the amount of rainfall an area receives,
and the amount of biomass it produces (think about the tropical rain forest
versus the extreme desert). Nutrient availability is also important. Not only
do nutrients need to be in the soil, but also they need to occur in a usable
form. For example, calcium may be present in the Texas hill country soils, but if the pH is
too high (above 7) the calcium may be bound to other minerals and unavailable
to the plants. Thus the pH of the soil (whether it is acidic like New England
or alkaline like the Texas
hill country) affects whether some nutrients are available for plant use.

Light and nutrient availability primarily
limit the productivity of aquatic environments. Water easily absorbs light.
Thus while it is light at the surface of a lake or the ocean, below 20 meters
it becomes very dark. In contrast, too much light can raise the temperature,
overheat the plants and kill them. Thus in the tropics primary productivity is
greatest a few meters below the surface where light is still available and the
temperatures are cooler.

ll. secondary productivity

Secondary productivity is defined as the
rate at which consumers (such as herbivores) convert food into their own
biomass. Thus it is the weight or bulk of energy in the animal's body. Since we
know that energy transfer is far less then efficient, we can see that there is
a lot less secondary productivity than there is primary productivity. This is
especially true since we look at different categories of secondary producers.
Herbivores like zebras, and carnivores like lions are endotherms. This means
they spend a tremendous amount of energy maintaining their own body temperature.
This is compared to ectotherms (such as snakes) that do not regulate internal
body temperature and require far less energy.

lll. ecological pyramids

Ecological pyramids (see Figure 54.11-54.14
in Campbell)
are a useful method to describe how much energy is transferred up through the
trophic levels. Anywhere from 5 % to as much as 20% of the productivity of one
level can be transferred up to the next trophic level. Typically pyramids
become very small at the top as the biomass is contained in a few larger
animals. There are usually 5 or fewer top predators in any ecological pyramid-
while the bottom of that same pyramid contains millions and millions of plants.
One of the most important lessons to be learned from these pyramids is the fact
that many more individuals can exist if they eat lower on the food chain. An
acre of land can feed one cow that can feed one person for a year. That same
acre of land can feed 22 people for a year if it is planted in foods (for
example grains, and vegetables) that people eat directly. As more and more
people go hungry in the world, there are very compelling reasons to limit our
intake of meat and make more food available.

It is important to reemphasize the fact that
nutrients often act as limiting factors in the environment. McNaughton (1988)
studied grasslands in the Serengeti and found that areas with high levels of
magnesium, sodium and phosphorous supported more herbivores than areas with low
mineral concentrations.

Minerals do not dissipate like other
resources but tend to clump, and accumulate in individuals or specific species.
This creates a "pool" of resources. Flux rate is defined as the rate
at which these resources move from one pool to another pool in the environment.
This is the theory behind using plants, such as mustard plants, to remove
minerals or hazardous materials from the environment. By growing mustard plants
on a particular patch of toxic soil, heavy metals can be absorbed by the
plants, thus shifting the pool of resources from the soils into the plants. In
the case of heavy metals, the plants can now be transported away from the site
and the environment can be made safe to grow other crops or simply as a place
for people to live. This process of using plants to removing radiation was used
in Chernobyl in
the 1980's after the nuclear power plant leaked contaminants into the
surrounding habitat.

ll. forces that move nutrients

Factors that affect the rate at which
minerals are transported include meteorological, geological and biological
factors. Meteorological factors are those caused by minerals being dissolved in
rain, snow, atmospheric gas and dust. Acid rain in a forest or on a lake would
be an example of meteorological movement. Geological movement would include
surface and subsurface drainage. An example would be minerals that are pulled
down into the soil after a rain. Biological movement includes the movement of
minerals through animals via digestion, predation, or the accumulation of
minerals in the animal's body.

Nutrient turnover rates can vary
dramatically between biomes. In rainforests, the turnover rate can be a short
as 10.5 years. In contrast the turnover rate in the taiga of the Soviet Union it may take as long as 42.7 years. One good
way to study nutrient rates and flow patterns is to use radioactive tracers to
label the elements as they move through the environment.

There are two basic types of nutrient
cycles. Local cycles have no mechanisms of long distance transfer. A good
example would be the turnover of resources in a pond or lake. Global cycles are
those that have an interchange with the atmosphere or the ecosystem. Minerals
that melt off a mountain and run into a river would be a good example. Global
cycles especially apply to the movement of nitrogen, carbon, and oxygen, as we
will see next .

lll. carbon cycle

The levels of atmospheric carbon are
normally quite low (0.03%). Autotrophs incorporate some of this carbon as
biomass via photosynthesis- (about 1/7th of the atmospheric CO2). Plants return
some of this to the atmosphere via respiration and decomposition. Fires
increase the rate significantly.

Volcanoes significantly increase atmospheric
carbon levels, while seasonal fluctuations exist as well. The rates of CO2 in
the atmosphere decrease significantly in the Northern Hemisphere summers while
increasing in the winter. Since there is more land in the Northern Hemisphere,
photosynthesis ties up CO2 in the summer. Photosynthesis rates decrease in the
winter but cell respiration rates by plants are still high. Cellular
respiration by animals appears to have a minimal effect on the rates of
atmospheric CO2 (remember that animals make up less than 1% of global biomass).

lV. nitrogen cycle

The nitrogen cycle is a global cycle. While
some of the details appear complicated, the cycle itself can be broken down
into 5 basic steps: 1) nitrogen fixation whereby bacteria can take nitrogen
from the air and reduce it to ammonia. This is only carried out by certain
bacteria such as Rhizobium, and a few algae, 2) nitrification where species of
bacteria such as Nitrococcus and Nitrobacter can take the ammonia from the soil
and convert it into nitrates- a usable form of nitrogen, 3) assimilation where
plant roots assimilate nitrogen as nitrates. Animals can then assimilate the
nitrates by eating the plants, 4) ammonification, where the decomposition of
plants and animals and the release of animal waste form ammonia in the soil. Detritivores such as bacteria and fungi out this process.
Since ammonia is not generally used by plants, however, valuable stores of
nitrogen are now unavailable, 5) denitrification, the reduction of nitrates
into gaseous nitrogen. Note that the process of denitrification by bacteria is
almost the reverse process of nitrogen fixation.

Nitrogen in the soil and water is more
important to organisms (in terms of its short-term availability) than
atmospheric nitrogen that is abundant, but unavailable. Atmospheric nitrogen is
important on a evolutionary time scale, however, as
quantities of usable nitrogen move between the air and the terrestrial
environment. Nitrogen availability is often the critically limiting factor that
affects individual species and population cycles. In modern agriculture vast
amounts of biomass are removed from the soil. With it goes the nitrogen that is
trapped in the plant. Thus the land is stripped of its nitrogen and farmers
must add nitrogen back to the soil in the form of fertilizers to renew the
land. By adding additional fertilizer, modern humans have doubled the amount of
nitrogen input in the terrestrial portion of the nitrogen cycle. Much of this
added fertilizer is then lost again through run off and erosion. The result is
eutrophication as vast amounts of nitrogen are dumped into localized water
sources. Algal blooms and other destructive consequences often follow as we
will see below when we look at human impacts on the environment.

V. phosphorus cycle

Phosphorus is different from other nutrient
cycles in that phosphorus fluctuates between geological and biological pools.
It is a simple cycle, however, because it does not contain an atmospheric
component. The Earth's crust is the primary source of phosphorus; it tends to
stay localized and cycle quickly. The exception is when phosphorus is carried
off of the land by erosion and runoff, and ends up in the sea as sediments on
the ocean floor. Geological events such as uplift and the movement of the
earth's plates may eventually redeposit the phosphorus back into the
terrestrial environment.

The most important form of phosphorus is phosphate, which plants can easily
absorb from the soil. They do this so quickly that often soils are depleted of
their phosphorus. Animals then obtain their phosphorus by eating other
organisms. Animals excrete phosphorus as a waste product in their urine and
feces. It also returns to the soil when plants and animals decompose.
Phosphorus is the limiting nutrient in aquatic environments where plants can
assimilate phosphorus even faster than terrestrial environments.

Vl. sulfur cycle

Human activity has had more effect on the
sulfur cycle than any other nutrient cycle. Sulfur is important because it has
a direct effect on soil pH levels. Many nutrients that may occur in soil are
chemically unavailable to plants if the pH level of the soil is too high or too
low.

Much of the sulfur on the planet is bound up
in geological deposits of organic matter such as coal, peat, oil, and in
organic matter such as rocks. Weathering of these materials releases sulfur in
a salt solution. Volcanoes and decomposition, especially in wetland ecosystems,
releases sulfur in a gaseous form of hydrogen sulfide (H2S). Hydrogen sulfide
quickly oxidizes into sulfur dioxide (SO2). Sulfur dioxide in turn is soluble
in water, and chemically converts into a weak sulfuric acid (H2SO4). Some
bacteria can now use this sulfuric acid and convert it back into hydrogen
sulfide, which starts the process all over again.

Read Concept 54.5 "Human
Impact on Ecosystems" on pp. 1200-1206

5.5 - Human Impact on
the Environment

Humans have a tremendous impact on the world
around us. Our impact is due to a number of factors including our sheer number,
our distribution around the globe, our use of resources in different
environments, and the outcome of our conspecific and interspecific
interactions. The three examples shed some light upon the consequences of our
actions.

l. human impact on food webs

DDT was first created in the late 19th
century and its usefulness as a pesticide was first recognized in 1939. It was
used in World War II and the Korean War to treat lice amongst the troops in the
foxholes and trenches. In the 1950's truck regularly came through neighborhoods
in Texas (Austin) spraying DDT on the cars, children,
and the foliage to control mosquitoes. By 1970, the global production levels of
DDT peaked. Most industrialized countries eventually realized its harmful
effects however, and DDT use was banned in the U.S. in the mid 1970's. DDT is
still banned for use in the United States
but is currently manufactured in south Texas
to sell for use in Mexico.

Why is DDT so destructive? First of all, it
does not decompose easily. DDT can last for 10 years or longer in the soil-
long after the "pest species" has died or become resistant to it. DDT
has low solubility in water, but high solubility in fats and lipids. Most fats,
in any environment, are found in living tissues of organisms. Thus DDT has a
great affinity for living organisms and tends to concentrate (biological
amplification) in the cells and tissues of individuals as one organism eats
another. Once DDT works its way up the food chain, it ends up in organisms such
as birds where is causes eggshell thinning. Egg shells become so delicate that
the weight of an adult bird brooding its eggs will
cause the eggs to crush. DDT ravished the Peregrine falcon populations in the
1960s as breeding pairs failed to produce young year after year.

ll. acid rain

As stated earlier, humans have had a massive
effect on the sulfur cycle. While we have increased the emissions of carbon
dioxide and nitrogen by 5-10 percent (with disastrous effects), we have
increased sulfur emissions by 160%, primarily through modern industrial
activity. Two activities that emit the greatest amounts of sulfur dioxide (SO2)
are smelting of non-ferrous ores, and the burning of coal and oil. Typically
the rocks that overlay deposits of oil and coal are also rich in sulfur. Mining
activities (such as strip mining) disturb these layers of rock and soils
exposing them to weathering agents such as air (which oxidizes them) and water.
Erosion then carries off the sulfur-laden soil. Mining can pollute lakes,
rivers and streams hundreds of miles away from the original source.

Smelting activities that start off as a
local problem around the mine or smelter soon turn into a widespread disaster.
For example, to avoid local polluting, smelters in Anaconda, Montana and other places now build taller
smokestacks. Prevailing wind currents can now carry the sulfur emissions to
other states and other countries, spreading the effects of the gas.

Gaseous sulfur can mix with rainwater,
resulting in acid rain. Recall that natural rain water varies in its acidity
and can be as low as 5.6 (recall that a neutral pH is 7.0). Most plants do best
in soils with a neutral pH but can withstand slightly acidic or alkaline
conditions. Acid rain has been measured as having a pH of 4.1 to 4.5. This is
over a 100 times more acidic than natural rain water. (Recall the pH of milk is
6.6 and vinegar is 2.7)

What are the effects of acid rain? Most
studies have focused on aquatic ecosystems where organisms are extremely
sensitive to changes in pH. In some lakes harm begins when the pH levels drop
below 6. Direct exposure to acid rain damages fish such as rainbow trout and
brook trout. While adult fish may be able to withstand fluctuations in pH,
small fry (newly hatched fish) are more sensitive. Thus an acid rain during the
breeding season may kill off all the young and decimate a species.

Acid rain causes minerals that are toxic
organisms to leach out of the soils surrounding the lakes and rivers and flow
into the water system. Fish and aquatic organisms that do not die from the
acidic water can be killed indirectly through aluminum, mercury and lead
poisoning.

Terrestrial systems also suffer when acid
rain causes "tree die back". Trees waste away from the inside out as
the external foliage and branches die off. Branches harmed by the initial rains
fail to leaf out the following spring, and the entire tree eventually dies due
to lack of foliage (needed for photosynthesis). Trees weakened by acid rain are
more vulnerable to insect attack, disease and harsh weather. Many scientists
suspect the maple groves of Vermont and Southern Canada have been experiencing tree die back
since 1980.

Countries that are most vulnerable to the
effects of acid rain are those areas of the world composed of pre-Cambrian
Shield bedrock. These bedrocks of granite and quartz found throughout North
America and Scandinavia are naturally acidic
and lack the ability to neutralize acid rain. Strata made of limestone, such as
the Hill Country region of Central Texas, are
naturally alkaline and can even benefit from the additional acidity. Prevailing
winds from the United States
often dump our smelting and industrial gases on Canada
and Northern Europe- the very regions most
vulnerable to harmful emissions.

lll. eutrophication

Eutrophication is the enrichment of water
with excess nutrients, primarily phosphorus and nitrogen. Recall that lakes
that are low in nutrients are called oligotrophic whereas lakes rich in
nutrients are called eutrophic. Increased nutrients lead to swift agal blooms
as the algae take the available nutrients and use them to rapidly reproduce.
This in turn increases water turbidity, decreases the levels of O2 in the
water, and decreases the water suitability to many indigenous organisms such as
fish.

There is wide spread global variation in lake eutrophication levels. Seventy-five percent of Canadian
lakes are still oligotrophic. In contrast, 70% of the U.S. lakes may be dangerously eutrophic.2 This is most likely due to the fact that farms surround many
smaller lakes in the United
States, and the farm run off that wind up in
the lakes contain excess fertilizers.

Eutrophication in rivers is harder to measure since nutrients quickly wash
away. Man made reservoirs on lakes show higher levels of eutrophication than
natural lakes. The degree of human induced eutrophication coincides with the
areas of highest human population density. Humans exacerbate eutrophication
through 4 pathways:

1) urban
waste (ex. detergents).

2) rapid
industrialization leading to industrial waste build up.

3) intensified
use of fertilizers in agricultural settings. These fertilizers tend to have a
clumped distribution.

4) distribution
of livestock and their waste.

There are two general methods for
controlling eutrophication. The first one is preventative:

1) treat
waste water to remove phosphorus and nitrogen.

2) divert
waste water from natural areas.

3) management
to limit water and fertilizer usage. Remove phosphates from detergents.

Secondly, corrective measures can be takes
to clean up those lakes and natural areas that are already polluted:

1. List the types of abiotic factors that might influence living organisms
within an ecosystem.

2. Describe using a series of simplified
chemical reactions what happens to the energy from sunlight has it
"flows" from species to species within a community. Label each
trophic level in this food chain.

3. Draw a sample food chain and trophic web.
Explain how the two differ from each other.

4. Explain how gross primary productivity is
related to net primary productivity. How does the concept of net primary
productivity relate to standing crop biomass?

5. List the five ecosystems that have the
greatest primary productivities, and explain why their productivities are high.

6. Explain what happens to energy in a green
plant when the plant is eaten by a herbivore; i.e. how
is the energy used by the herbivore.

7. Draw a pyramid of productivity. Label
each level. Why does this have a pyramidal shape? Why is this
pyramid shape sometimes inverted when standing crop biomass is plotted
for aquatic ecosystems?

8. Draw and label a generalized model of a
biogeochemical cycle.

9. Explain what happens to nitrogen, sulfur,
or phosphorus in its own cycle.

10. Describe what happens to water and
mineral cycling in forested watersheds when these are deforested.

11. How does erosion trigger the process of
eutrophication?

12. Explain how biomagnification can lead to
high concentrations of persistent pesticides and heavy metals in upper trophic
level predators.

13. Draw a simplified sketch of the carbon
cycle. Use this cycle to explain where the carbon comes from that is building
up in our atmosphere. How does this excess carbon effect global climate?